MAP - Detect a free sector on the disk

Syntax:

CALL 4029H
Input:

  • filename in FNAM (IY+1)

  • File Type in TYPE (IY+9)

Exit:

  • NSCT (IY+21) = Sector Number

  • NTRK (IY+22) = Track Number

  • The sector addressed with NSCT and NTRK was reserved іп internal allocation map (MAPAREA)

Registers used:

AF, BC, HL

Error handling:

  • A = 0 sector found

  • A = 7 no free sector (disk full)

This routine determines a free sector in the internal allocation map in DOS vectors (MAPAREA), which should previously be filled with the current allocation map from track 0, sector 15 of the diskette.

If a free sector is determined, the corresponding bit in the allocation map is set to 1.

Please note, however, that a final allocation has only taken place when the allocation map has been written back to the diskette.

The result (the sector address) is passed in the NSCT and NTRK fields of the DOS vectors. If you want to access the sector, e.g. with WRITE, you must first transfer this address to the SCTR and TRCK fields.

No disk access is performed by the MAP routine.

Example:

...
DI                  ; disable interrupts
LD (IY+11),80H      ; select Drive 2
CALL 4008H          ; and turn on
LD BC,50            ; 50 ms delay
CALL 4011H          ; load allocation map
CALL 4029H          ; determine free sector
OR A                ; error occured?
JP NZ,400EH         ; yes, to the ERROR routine
CALL 4017H          ; write back allocation map
CALL 400BH          ; turn off the drive
EI                  ; allow interrupts again
...

A free sector is determined and allocated on the diskette in drive 2. The sector address is passed in the NSCT and NTRK fields of the DOS vectors.